The invention relates to a paper or paperboard laminate composed of at least one bulk-promoting layer, here termed bulk layer, and on at least one side of the bulk layer at least a secondary layer, the secondary layer and bulk layer being joined to one another directly or indirectly over basically the whole of their surfaces facing one another. The invention also relates to a method for producing such a laminate.
One of the most important attributes of paperboard material when used as packaging material is its stiffness. The stiffness of a paper or paperboard laminate is proportional to its thickness raised to the third power. This ratio means that a considerable saving in material can be achieved by reducing the density of the less loaded middle layers in a laminate. The ratio has long been known, but one difficulty has been to produce sufficiently stiff and strong middle layers which at the same time are of a low density.
Corrugated paperboard is a classic example of a paperboard laminate with good flexural rigidity in relation to the density of the laminate. Due to microcorrugation of the bulk-promoting middle layer, relatively thin laminates can also be produced, which are not however regarded as satisfying the maximum demands made on packaging material. Thus the wave-shaped pattern can often be discerned, which reduces the aesthetic value of the material.
In xe2x80x9cWeyerhaeuser Paper Company introduces HBA (High Bulk Additive)xe2x80x9d, Elston and Graef describe the possibility of using chemically cross-linked fibres in paperboard material. By adding 10% HBA (High Bulk Additive) to the stock, the basis weight of the paperboard material can be reduced by 25%, with a sheet of the same flexural rigidity as a control sample without the addition of HBA. The thickness of the sheet can be retained, the density being reduced instead in one example from 705 to 500 kg/m3. Taber stiffness is shown to increase by approx. 40% with the addition of 15% HBA. However, this results in reduced tensile strength, approx. xe2x88x9225%. Admixing has been performed inter alia on a three-layer laminate, all the HBA being put into the middle layer.
WO95/26441 likewise describes the use of a chemically cross-linked fibre in paper laminate with two or more layers. The object of using the cross-linked fibre (HBA) is to achieve a construction of increased bulk while retaining the tensile strength. Paper material of low density (high bulk) normally gives lower tensile strengths. To reduce this negative effect of low density, the use is proposed of waterborne binders sirch as starch, modified starch, polyvinyl acetate and polyvinyl alcohol etc. It is proposed to use these binders in percentages of between 0.1 and 6% of the material""s weight. The flexural rigidity achieved is expressed in Taber units. If the same method is used for converting stiffness as described below under test methods, then the result in WO95/26441, Example 5, corresponds to a bending stiffness index of approx. 1.6 Nm7/kg3.
Dry forming in the manufacture of paper has been described in literature in a large number of articles and patents. In xe2x80x9cAn introduction to dry forming of paperxe2x80x9d, Tappi, 1978, pp. 3-6, amongst others, Swenson describes various techniques for forming a web using air as a dispersing medium for wood fibres. Here examples are given of products which are manufactured by dry forming, e.g. soft hand towels, stiff paperboard and masonite.
In GB 1,430,760 and GB 1,435,703 a forming technique is described for producing paper material with several layers. It is proposed inter alia to combine dry- or wet-formed layers with one another. It is proposed that consolidation of the sheet (consisting of several layers) is done by using binders, moisture and pressing at high temperature. Product attributes for dried-out products are characterized by high bulk, squareness (i.e. same properties in different directions of the sheet in a plane) and good dimensional stability. Furthermore, it is considered possible to achieve product attributes similar to conventionally formed paperboard. The manufacturing technique is considered to reduce investment costs, water and energy consumption.
In xe2x80x9cWhere research pays offxe2x80x9d, PPI, March 1977, pp. 42-26, Haas describes certain important product attributes for conventionally wet-formed and dry-formed paperboard. Haas describes the attributes of the dry-formed sheets as having an even surface with a lack of felt and wire markings and an approved tear strength. Stiffness is reported using numerical values for the various manufacturing techniques, but not commented on in the text. The dry-formed multilayer materials have not produced increased stiffness. In interpreting the document here it has been assumed that xe2x80x9cstiffness %xe2x80x9d or xe2x80x9cstiffness Xxe2x80x9d means the stiffness of the sheets in a transverse or longitudinal direction (TR or MR). In the event of conversion for better comparison between different materials, the bending stiffness index can be calculated as the geometric mean value of MR and TR (the square root of MR*TR), a maximum bending stiffness index achieved according to the values reported by Haas being approx. 1.2 Nm7/kg3. It is thus perceived here that dry forming techniques such as have been applied have not contributed to increased flexural rigidity. Haas also reports the basis weight and thickness of the different paper constructions, 550 k/m3 appearing to be the lowest density produced for the wholly or partly dry-formed constructions.
In xe2x80x9cDry forming of paperboard: a look at its history and technologyxe2x80x9d, Pulp and Paper, 54, 1980:4, pp. 120-123 Attwood reports on experiments inter alia with paper constructions which combine dry-formed and wet-formed layers. The results reported with regard to stiffness and thickness (at the same basis weight) point to great differences in stiffness in machine (MR) and cross machine direction (TR). The maximum stiffness converted as the square root of stiffness MR*TR was obtained for material which had been produced with wet-formed outer layers and dry-formed middle layers, no values in excess of 1 Nm7/kg3 having been achieved, however. Furthermore, Attwood reports various proposals for methods of designing a process which combines dry-formed middle layers with wet-formed outer layers. Attwood also reports the basis weight and thickness of the different paper constructions, approx. 600 kg/m3 appearing to be the lowest density produced for the wholly or partly dry-formed constructions.
U.S. Pat. No. 4,914,773 reports methods of producing stiff paperboard material by using dryly exposed fibres with a freeness of 500 CSF. The fibres which are to be formed into the middle layer in a sheet are to be dispersed in foam. This has the object of preventing them from being wetted with water to too great an extent. The addition of different types of binder such as latex, starch, gums etc. is specified as necessary preconditions for achieving adequate strength of the sheet. When the flexural rigidities reported are converted it is clear that the maximum bending stiffness index achieved is approx. 1.8 Nm7/kg3.
It has turned out surprisingly to be the case that by using fibres with a freeness of 550-950 ml CSF, preferably fibres with a freeness value higher than 600 ml CSF, at best higher than 650 but less than 850 ml CSF, and best of all higher than 700 ml CSF, in a bulk-promoting layer in the laminate, termed bulk layer below, in combination with a secondary layer on one or both sides of the bulk layer, a laminate can be obtained which exhibits very great stiffness. The advantage is also hereby achieved that the laminate has a lower density, and thereby lower material consumption compared with previously known paperboard laminates intended for the same type of use as the laminate according to the invention, such as material for packaging of liquid and solid foodstuffs and also for wrapping and packing industrial goods and other goods, or as an intermediate product for the manufacture of such material or other end products. A paper or paperboard laminate is presented by means of the invention with a bending stiffness index greater than 2.5 and lower than 14 Nm7/kg3, which is a bending stiffness index more than 2-7 times higher compared with multilayer paperboard conventionally produced today. At the same time, the laminate has sufficient strength in the bulk layer, which normally constitutes the middle layer in the laminate, to facilitate folding and subsequent creasing of the material. A particular advantage of the invention is that after folding it can be creased without obstruction both to and from the fold impression.
The bulk layer has a very low density, 50-300 kg/m3, preferably 70-200 kg/m3, at best 100-180 kg/m3 and a basis weight of 30-300 g/m2. According to a conceivable embodiment it has a basis weight of 40-80 g/m2, and according to another embodiment a basis weight of 70-120 g/m2. According to another aspect of the invention, the bulk layer has a thickness of 0.1-6 mm, preferably 0.2-1.0 mm, at best 0.3-0.7 mm.
Said secondary layer has a considerably greater density and tensile strength than the bulk layer, e.g. a density which is at least twice as great, preferably at least three times as great and best of all at least four times as great as the density of the bulk layer. Thus the secondary layer can have a density of 300-1500 kg/m3, preferably 400-850 kg/m3. The average thickness of the secondary layer/the individual secondary layers is typically only 3-20%, preferably a maximum of 15%, at best a maximum of 10% of the thickness of the bulk layer.
A laminate according to the invention consisting of one bulk layer and one secondary layer on at least one side of the bulk layer, preferably on both its sides, has a basis weight of between 50 and 500 g/m2. Within the said interval, a laminate composed according to the invention can have a basis weight which depends on the reciprocal relationships between the thicknesses and densities of the bulk layer and the secondary layers. Thus when the bulk layer is relatively thick, the laminate can have a basis weight of 75-400 g/m2, preferably 100-350 g/m2, at best 100-250 g/m2 or 90-200 g/m2. If on the other hand the bulk layer is relatively thin, the laminate can have a basis weight of 300-500 g/m2, preferably 350-450 g/m2. In other words, the secondary layer/secondary layers dominate weight-wise in this case. An intermediate case is also conceivable, when the laminate consisting of said layers has a basis weight of 200-400 g/m2, preferably 250-350 g/m2.
The tensile index of the laminate according to the invention can amount to 25-150 Nm/g, preferably 50-100 Nm/g.
During manufacture, the bulk layer is laminated using binders and with controllable pressure and time to said secondary layer with high tensile strength to form a laminate according to the invention. Lamination can be carried out advantageously at the same time as the bulk layer is consolidated. However, this is not a prerequisite, on the contrary, it is equally possible to first form and consolidate the bulk layer by drying, this then being laminated to the desired secondary layer.
Said bulk layer of low density can advantageously be produced by dry forming or by wet forming of chemi-thermomechanical pulp (CTMP) or another xe2x80x9cmechanicalxe2x80x9d pulp based on softwood fibres, e.g. TMP, with a high freeness. Dry forming is preferable from one aspect, it being possible to use any known technique for this, but regardless of the forming technique the freeness of the pulp should be higher than 550 CSF, preferably higher than 600 CSF and even more preferredly higher than 650 CSF, best of all higher than 700 CSF. A high freeness of the fibre material for said first layer ensures that the sheet can be pressed on dewatering and consolidation of the sheet without the density increasing to an undesired extent. Other raw fibre materials with high wet resilience can also be included in the bulk layer to a certain degree, e.g. chemically cross-linked fibres, which often have a slight dewatering resistance and high resilience after wet pressing, but are not to be preferred at least for cost reasons.
Further conceivable raw fibre materials are synthetic fibres, e.g. polyester, polyethylene and polypropylene fibres, which also exhibit a low resistance to dewatering and high resilience in the wet state. In a preferred embodiment, the raw material for the layer with low density for the bulk layer, which is normally to form the middle layer in the laminate, is selected wholly or mainly from mechanically produced, so-called high-yield pulps, i.e. pulps with at least 75%, suitably at least 80% wood yield, such as CTMP and TMP pulps for example, based mainly on softwood fibres, with the prerequisite that the pulps have the freeness values specified above.
Waste matter can also be added to the bulk layer up to 40% of the dry weight. Waste is defined here as reject paper or paperboard laminate product which has been flailed in a pulper and with mainly exposed fibres.
The laminate according to the invention is constructed in a preferred embodiment of three layers, with two or more than three layers being conceivable, said bulk layer preferably being laminated together with the secondary layers on both sides. However, it is conceivable for a secondary layer to be present only on one side of the laminate. This/these secondary layer(s) can be produced advantageously in the same plant as the bulk layer, but also manufactured separately in order to be laminated to the bulk layer in a separate installation.
No restrictive meaning is to be imposed by the expression xe2x80x9csecondaryxe2x80x9d layer. There can thus be further layers, e.g. barrier layers, on top of the secondary layer/layers, or between any secondary layers and the bulk layer. It should also be understood that the secondary layers/surface layers; secondary layer/surface layer can be coated to improve printability. Typically the preferably coated layers are coated in turn with a plastic layer or are intended to be plastic-coated if the laminate is an intermediate product, in order in a manner known in itself to make the laminate waterproof and heat-sealable for it to be able to be used for liquid packagings. The secondary layers/surface layers can thus have several functions in combination with the bulk layer, such as making the laminate impermeable to liquid and steam, heat-sealable and giving the desired tensile strength and bending strength.
According to an aspect of the invention, at least one secondary layer of the laminate is permeable to steam, this secondary layer being formed by stock with a dewatering resistance higher than 20xc2x0 SR, preferably higher than 25xc2x0 SR but not higher than 65xc2x0 SR, preferably not higher than 40xc2x0 SR, to ensure the removal of water on thermal drying. It is presupposed that the laminate on this side of the bulk layer does not contain any other layer either which is impermeable to steam during the drying process. The permeable surface layer/layers are best constituted by wet-formed paper with a xe2x80x9cGurleyxe2x80x9d air permeance of more than 2 xcexcm/Pa*s which is preferably produced as a chemical pulp of softwood and/or hardwood.
To achieve necessary strength in the thickness direction (z-direction) and with regard to flexural rigidity, binders are added, preferably latex binders, in a quantity of 1-30% of the weight of the laminate, suitably 5-30%, preferably 7-30% and even more preferredly 10-20% calculated as dry weights. These polymer binders can be added dissolved and/or dispersed in water by applying spray directly to the bulk layer and/or the secondary layers in order to be transferred with these to the bulk layer and penetrate it. Various types of coating systems can also be used to add binders to the secondary layers. Coating can thus be carried out using blade coaters, directly or indirectly using roller coaters.
According to an embodiment of the invention, high flexural rigidity and good strength in the thickness direction are achieved in the laminate with relatively low basis weights of the bulk layer even with low percentages of binders, i.e. binder percentages preferably of latex binders of as little as 1-5% of the weight of the laminate, preferably 2-5% of the laminate""s weight. The bulk layer should here have a basis weight of 30-100 g/m2, preferably 30-80 g/m2.
Without limiting the invention to a certain theory, one reason that low percentages of binder are sufficient with low basis weights of the bulk layer is believed to be that the binder does not follow with the water to the surfaces to the same extent in connection with drying when the laminate thickness is smaller. As binders are a relatively expensive raw material in the laminate, every % unit reduction of binders signifies major savings.
Latex is best used as a binder in the bulk layer in the above specified percentage and another binder, e.g. starch, carboxymethyl cellulose or gums to achieve bonding between the secondary layer and bulk layer.
Suitable binders for penetrating the bulk layer can be diluted in water, i.e. are soluble or dispersible in water and are selected preferably from the group consisting of water-soluble polymers, or aqueous dispersions of polymers, such as polyvinyl acetate, polyvinyl alcohol, polyacrylates, polyacrylic acid, polyethylene, acrylamide, polystyrene and maleic acid derivative in the form of homo- and copolymers of said polymers, or possibly from the group consisting of starch, carboxymethyl cellulose and gums, the latter group being particularly suitable for use with the aim of achieving bonding between various fibre-based layers.
Furthermore, the different layers with fibres can contain hydrophobic rendering additives such as AKD adhesive (alkyl ketene dimer adhesive), resin glue, silicon-based and fluorinating substances etc. in a quantity corresponding to a maximum of 2% of the respective layer weight.
According to one aspect of the invention, at least one of the secondary layers can have been formed and pressed in a separate stage/separate stages before being laminated to the bulk layer. The objective is to increase the dry content and increase the tensile strength of the surface layers separately without pressing said bulk layer to such an extent that its low density is lost. Consolidation of the multilayer laminate with the help of binders can thus be carried out in pressing conditions which are not determined by the need for the secondary layers to be pressed for consolidation. A drying stage with heat can possibly be inserted to adjust the dry content to the desired level prior to combination and joining of the various layers.
To distribute the binder in the bulk layer, which normally forms the middle layer, the material is best pressed in one or more press nips before drying. The pressing is carried out in such a way that the density of the bulk layer does not exceed the limits set above after drying. Pressing on lamination can be carried out advantageously between rollers or belts without removing the water. The quantity of water which is to be dried away by the use of heat can also be reduced by using a press section of the type which is used in conventional paper machines, or as a combination. If water is removed in the pressing operation, however, there can be a risk of losing the binders, which constitutes an environmental and economic disadvantage.
Following the press, the laminate is dried in conventional drying equipment such as a cylinder dryer with or without dryer wire/felt, an air dryer, metal belt etc. Following drying or during a suitable break in the drying process, the material can be coated. Alternatively, secondary layers are used with one or two coated surfaces.
To further reinforce its attributes as packaging material, the laminate can be completed by layers which can be surface layers or intermediate layers, and which constitute barrier layers in the form of films of various polymers, polyethylene, polypropylene, polybutene, polyester, polyvinyl and/or vinylidene chloride, polyvinyl alcohol, polyethylene vinyl alcohol co-polymer, ethylene vinyl acetate co-polymers and cellulose esters in one or more layers or aluminium foil or metallized polymer film. Said barrier layers can also be placed directly against the porous bulk layer, the binder in this case contributing to giving the final laminate the necessary strength. In this case, however, the laminate must be permeable to steam on one side of the bulk layer to ensure the removal of steam. In the event that the barrier layers are intended to be laminated directly to the bulk layer, it is also essential that it does not have a yield point which is lower than the surface temperature of the drying cylinders, normally 130-180xc2x0 C.
The laminate according to the invention is used preferably for food packaging or packaging for consumer products of various types etc. Also dried laminate of said type serves well as protection during the storage and transportation of goods for industrial use.
The new paper laminate has a major advantage in that it produces less waste of raw materials to achieve a certain level of stiffness in a packaging material. This means reduced costs and/or reduced environmental pollution in relation to the transportation of timber raw materials and end products. Total energy consumption is thereby reduced in manufacturing the paper laminate according to the invention compared with conventional paperboard manufacture. The consumption of electrical energy is also reduced in manufacturing TMP or CTMP raw fibre materials with the desired attributes. These raw fibre materials are also considerably cheaper to manufacture than chemically cross-linked fibres and chemically produced softwood pulps according to the sulphite or sulphate processes.
Further aspects and features of the laminate according to the invention and the method for producing the laminate are evident from the following patent claims and the following description of some conceivable methods of producing the laminate and a number of conceivable embodiments of the laminate together with experiments carried out.